You don't think a single piece of hardware with 2 extra batteries, 2 extra wireless radios, and 2 ports adds any costs? It made the unit more expensive, less ergonomic, and/or they passed on hardware features they otherwise could have implemented.
Lifetime Sales Prediction - 6/29/2013
Wii U - 38 million
XBOX One - 88 million
Playstation 4 - 145 million